A small marine snail common to Australia uses its shell as a selective 'lampshade' to diffuse and amplify the light it emits and deter predators, researchers have discovered.
Marine biologists have known since 1988 that the snail, Hinea brasiliana, produces light - a phenomenon known as bioluminescence - when knocked around. Research published today in the Proceedings of the Royal Society B describes how the snail uses its shell to exaggerate its green glow.
The snail is part of the Planaxidae family, also known as 'clusterwinks' after their habit of grouping together in crevices at low tide. It lives on the rocky foreshore is found on coasts from central Queensland down the east coast to South Australia.
Bioluminescence is a common communication method in open water molluscs such as squid, says Dr Nerida Wilson, a marine researcher at Sydney's Australian Museum and an author on the paper. Bioluminescence in marine snails is much rarer, being found in just a handful of groups, she says.
Hinea brasiliana produces light from two patches of cells on a part of its body that is always hidden underneath its shell. The shell is hard, opaque and yellow-coloured, leaving researchers puzzled about how the snail could get its message across.
Light refracted in crystalline shell structure
A closer look at the shell revealed that it allows all wavelengths of light to pass through except the blue-green wavelength of the light that the snails produce. Instead, that light is retained and scattered around the shell.
"So instead of having two little spots where the bioluminescence is produced, the whole shell lights up. The small signal that it begins with is spatially amplified so that organisms around it can see it," says Wilson. It also means the snail can emit light while safely tucked up in its shell.
She's not sure yet how the shell traps just the bioluminescent wavelength and scatters it so well, but that it is probably associated with the crystal structure of the shell's calcium carbonate rather than the shell shape shape or colour.
The snails only glow when mechanically stimulated, such as being knocked around or bumped into by a fast-swimming animal. Upon collision the snail gives off a series of short, rapid flashes, which appear like a glow.
Wilson says it might be that a whole cluster of H. brasiliana would glow if one was bumped into while sheltering at low tide.
She is now looking at other members of the Planaxidae family to see whether the snail's bioluminescent relatives also have shells matched to their wavelengths.
"I'm trying to create an evolutionary framework to test how these things have evolved. I'm very interested in whether there has been coevolution between the shell and the bioluminescent system", she says.